Automatic torque limitation control

ABSTRACT

The invention is a torque control on a variable displacement pump. A pilot-operated valve controlling the flow to and from the pump displacement cylinder allows system pressure to flow to the displacement cylinder or drains the displacement cylinder to reservoir. The pilot-operated valve is controlled by opposing servos, both sensing pump system pressure; one of the servos urging the valve means toward the drain position with the assistance of a biasing means. The bias-assisted servo is connected to a variable relief valve, the variable relief valve being connected to the pump swash plate in a manner causing the relieving pressure level to increase as the pump swash plate decreases its displacement whereby the pilot-operated valve means maintains a decreasing displacement level at higher system pressures so as to achieve constant torque.

United States Patent [1 1 Martin Feb. 4, 1975 AUTOMATIC TORQUE LIMITATION CONTROL [75] Inventor:

[73] Assignee: The Cessna Aircraft Company,

Wichita, Kans.

[22] Filed: Sept. 11, 1973 [21] Appl. No.: 396,317

Robert J. Martin, Hutchinson, Kans.

[52] US. Cl. 417/222 [51] Int. Cl. F04b 1/26 [58] Field of Search 417/213, 218, 222, 212; 91/506 [56] References Cited UNITED STATES PATENTS 2,835,228 5/1958 Parret et a1. 417/222 3,017,750 1/1962 Kempson 417/218 3,250,227 5/1966 Kouns 417/222 3,667,867 6/1972 Boydell et al 417/222 3,669,570 6/1972 Himmler 417/222 3,768,928 10/1973 Miller et al. 417/213 FOREIGN PATENTS OR APPLICATIONS 1,126,873 9/1968 Great Britain 417/222 6/1965 Great Britain 417/218 9/1959 Great Britain 417/218 Primary Examiner-William L. Preeh Assistant ExaminerGregory Paul LaPointe [57] ABSTRACT The invention is a torque control on a variable displacement pump. A pilot-operated valve controlling the flow to and from the pump displacement cylinder allows system pressure to flow to the displacement cylinder or drains the displacement cylinder to reservoir.

, The pilot-operated valve is controlled by opposing servos, both sensing pump system pressure; one of the servos urging the valve means toward the drain position with the assistance of a biasing means. The biasassisted servo is connected to a variable relief valve, the variable relief valve being connected to the pump swash plate in a manner causing the relieving pressure level to increase as the pump swash plate decreases its displacement whereby the pilot-operated valve means maintains a decreasing displacement level at higher system pressures so as to achieve constant torque 2 Claims, 2 Drawing Figures PATENTED 4.1975 I 3.864.063

sum 1 or 2 PATENTEDFEB 3,864,063

SHEET 2 OF 2 ZOOO' ISOO- PSI IOOO- 1 AUTOMATIC TORQUE LIMITATION CONTROL BACKGROUND OF THE INVENTION This invention is in the field of variable displacement pumps which supply varying load requirements for both mobile and stationary hydraulic systems. More particularly the system provides a means for controlling the variable displacement pump at a constant input torque so as to better match the size of the prime mover lo the pump. One application of a constant torque pump would be on a mobile unit such as a backhoe wherein it is desirable to move all functions rapidly at low pressure, but during heavy digging require the pump to reduce its stroke so as not to require a torque level exceeding some predetermined maximum, while providing pressures in excess of those achievable by the power source at maximum pump displacement. On ordinary fixed displacement pump systems, the pump must be large enough to give the required speeds under low pressure levels while during the slower moving digging phase the full flow must be pumped at high pressure with a good portion of that high pressure flow being wasted over a relief valve. This type of system requires a large horsepower engine to drive the pump and a great deal of heat is put into the system causing not only power waste but additional problems with regard to cooling. With a torque-compensated system similar to the present invention, a much smaller engine or prime mover can be utilized since during high load phases the displacement is reduced as the pressure level rises. This type of system also has an application with an electric lift truck where power conservation and efficiency are even more important. With the present invention, heavier loads can be raised at the expense ofspeed without exceeding the available torque. A smaller electric motor can be used in such a system since the torque requirements of the pump will remain within the prescribed limits and the motor speed will stay within an efficient range. Such a system prevents the motor from being pulled down to a low rpm under heavy load resulting in a wasteful high amperage draw or motor damage.

US. Pat. No. 3,250,227 describes a method of maintaining constant input torque by the use of a pilotoperated valve spool controlling flow to and from the displacement cylinder, the spool being basically pressure responsive against the bias ofa compression spring which gradually increases the spring force as the swash plate moves towards minimum flow position. Another teaching of a constant torque system is found in US. Pat. No. 3,384,019 which utilizes a variable orifice tied to the cam plate position with a constant flow rate into a sensing chamber with the sensing chamber pressure controlling a pilot-operated valve which in turn controls the displacement cylinder of the pump. This system is dependent upon a constant flow rate to the sensing chamber while the torque-compensated system of the present invention is not so limited.

BRIEF SUMMARY OF.THE INVENTION In the present torque-compensated system a variable relief valve is utilized to control the servo pressure which operates the pump displacement cylinder. The level at which the relief valve relieves is hydromechanically tied to the swash plate position whereby the relieving pressure level is increased as the swash plate decreases its displacement thereby requiring a higher system pressure level before the control again adjusts the swash plate position.

Therefore the principal object of the present invention is to provide a new and simplified automatic torque input control on a variable displacement pump.

Another object of the present invention is to provide a more sensitive torque-compensating system.

Another object of the present invention is to provide a torque-compensated system used in conjunction with a conventional pressure compensated system.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the following drawings in which:

FIG. I is a partially symbolic circuit of the torquecompensated system with portions of the pump displacement cylinder shown in section; and

FIG. 2 is a graphic showing ofa typical pressure flow curve for a torque-compensated pump.

While the invention will be described in connection with a preferred embodiment it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents within the spirit and scope of the invention as defined by the appended claims.

Turning to FIG. 1, a conventional axial piston variable displacement pump is generally identified by reference numeral 10. The invention can also be used on other types of variable displacement pumps such as vane types. Pump is driven by a prime mover, not shown, through shaft 17. Various types of internal combustion engines as well as electric motors can be used. The pump 10 includes a rotating cylinder block 12 on a stationary valving plate 16. The cylinder block 12 has a plurality of pistons ll slidable therein in a reciprocating manner due to the camming action of tiltable swash plate 14. Swash plate 14 is biased towards the maximum flow position, as indicated in the drawing, by spring means 15 or other conventional biasing means such as offsetting the swash plate pivot 13 or advancing the valving slots, none of which are shown in the drawing. Both of these last mentioned methods, as well as the previously mentioned structure of the pump, are all well known in the art and therefore not shown and described in detail.

The swash plate 14 is positioned by the single acting displacement cylinder and the force of biasing spring 15. The cylinder 20 includes a piston 22 with a laterally disposed partition wall 25 located midway therein. The partition 25 includes a bore therethrough for receipt of a hollow plunger 26 having a hole 30 in the outer end thereof. The plunger 26 has an inner passage 24 connecting hole 30 to chamber 33. Attached to the rear end of plunger 26 is a flange 58 which is in contact with compression spring 34. With the swash plate in its maximum flow position, plunger 26 is fully retracted to the left with flange 58 in contact with lug 32. Plunger chamber 33, which is open to reservoir through hole 30, is connected to the pump discharge line 36 via sensing line 56 and across orifice 50. This allows a flow through chamber 33 limited by restriction 50 and hole 30. The pressure in chamber 33 is sensed in chamber 46.

The return line 35 for the pump is shown connected to a reservoir, however, it could also be connected to the discharge side of a hydraulic motor being driven by pump 10. i

The control of the displacement cylinder is provided by a pilot-operated spool valve 40 including a spool 41 which is spring-biased to the right by spring 45. Pressure in servo chamber 44 at the right end of spool 41 senses the pump discharge pressure through sensing line 51 urging the spool 41 to the left against spring 45. Pressure in'servo chamber 46 urges the spool to the right, sensing the pressure in plunger chamber 33 via line 52 and 56. In a deficient pressure or start-up position, spool 41 is forced to the right by spring 45 opening groove 43 in the spool to cylinder chamber 21 and drain line 55, thereby allowing the displacement cylinder to fully retract to the maximum pump displacement position} With the pump at maximum displacement or full stroke, system pressure begins to build in discharge line 36. This pressure is sensed through line 51 to chamber 44 eventually through overcoming spring 45 and moving valve spool 41 to the left. This leftward movement of land 42 opens up servo chamber 44 to line 54, thereby charging fluid into the displacement cylinder 20 and decreasing the displacement of the pump. As the displacement piston 22 moves to the right, the partition wall 25 attempts to close hole 30. An increasing pressure in chamber 33 acting upon plunger 26 causes the plunger to follow the movement of piston 22 as in any servo mechanism. The pressure rate of increase, established in chamber 33 and transmitted to chamber 46 is governed by the travel of plunger 26 as it causes spring 34 to compress and gain a load based upon desired characteristics. While compressive .spring 34 is shown as a single spring, it may also be desirable to utilize a two-stage spring with the second stage coming into compression after the first stage is partially compressed so as to increase the spring gradient. This double spring gradient is illustrated in FIG. 2 by the lines 64 and 66 which very closely approximate a constant torque curve 62. This particular spring gradient can also be achieved by the specific design of a single spring.

. OPERATION When pump 10 is initially started, the swash plate 14 will immediately move to its maximum flow position since cylinder chamber 21 is open to drain via lines 54 and 55 due to the position of spool 41 to the right under the sole influence of spring 45. With the pump at full stroke, external loading of theoutput will cause the output flow to pressurize. This build-up in system pressure is sensed in chamber 44 and when it exceeds the force of spring 45, the spool 41 shifts to the left to its flow-blocking position, as illustrated in FIG. 1. In viewing the pressure flow curve of HO. 2, portion 60 of the curve illustrates the initial pressure build-up in the start-up configuration. When the pump discharge pressure reaches the 500 PSI level, which is only used as a matter of illustration, the pump displacement begins to decrease since the level of constant torque has been reached. At this point on the curve, the pressure in chamber 44 has caused the valve spool 41 to shift further to the left, opening passage 54 and allowing flow into the displacement cylinder chamber 21, thereby causing piston 22 to extend and decrease the pump displacement.,ln these initial start-up conditions the pressure in servo chamber 46 is essentially zero since the restriction 50 is greater than the hole in the end of plunger 26. As piston 22 moves to the right, de-stroking the pump, the hole 30 begins to orifice the flow, causing pressure to momentarily build in chamber 33. This momentary pressure build-up causes plunger 26 to shift to the right, again opening hole 30 to drain. For each incremental movementof piston 22 to the right. there is an increased pressure in chamber 33 due to the increased spring force of spring 34, thereby causing a pressure increase in servo chamber 46. The affect of the pressure increase in chamber 46 requires a greater system pressure sensed in chamber 44 to further destroke the pump. Therefore, as the system pressure level rises, due to an increasing load, the pump 10 reduces its displacement as indicated approximately by curve 62. When the system pressure reaches its maximum level, hole 60 in plunger 26 opens the cylinder chamber 21 to pressure chamber 33 causing fluid to flow into cylinder 20 and stroke the pump back to zero, thereby producing line 63 in FIG; 2.

Having described the invention with sufficient clarity to enable those familiar with the art to construct and use it, I claim:

1. A torque-compensating control system for a variable displacement pump with an outlet port, the pump having a tiltable swash plate actuated by a fluid operated displacement cylinder normally biased toward its maximum flow position, the system comprising:

a pilot-operated valve means positioned in fluid communication with the displacement cylinder, pump outlet port, and drain, having a first position directing fluid pressure to the displacement cylinder and a second position draining the displacement cylinder to tank;

a first pilot servo urging the valve means towards its first position;

a first sensing line connecting the first pilot servo to the pump outlet pressure;

a second pilot servo urging the valve means toward the second position with the assistance of a biasing means;

a second sensing line connecting the second pilot and variable relief valve means relieving the pressure in the second pilot servo, the relief valve means including a hollow pressurized plunger reacting against a spring means with its interior connected to the second pilot servo, the plunger having a first valve opening therein, the plunger passing axially through the displacement piston with the displacement piston acting as a valving means on the first valve opening, the spring force on the plunger increasing as the plunger follows the piston toward a minimum flow position causing the relieving pressure level to increase as the pump swash plate decreases its displacement whereby the torque output of the pump remains substantially constant at various pump discharge pressure levels.

2. A torque-compensating control system as set forth in claim 1, and a second valve opening in the plunger so positioned that when the maximum system pressure is attained, the second valve opening opens fluid communication between the interior of the plunger and the displacement cylinder thereby reducing displacement of the pump.

servo across an orifice to the pump outlet pressure; 

1. A torque-compensating control system for a variable displacement pump with an outlet port, the pump having a tiltable swash plate actuated by a fluid operated displacement cylinder normally biased toward its maximum flow position, the system comprising: a pilot-operated valve means positioned in fluid communication with the displacement cylinder, pump outlet port, and drain, having a first position directing fluid pressure to the displacement cylinder and a second position draining the displacement cylinder to tank; a first pilot servo urging the valve means towards its first position; a first sensing line connecting the first pilot servo to the pump outlet pressure; a second pilot servo urging the valve means toward the second position with the assistance of a biasing means; a second sensing line connecting the second pilot servo across an orifice to the pump outlet pressure; and variable relief valve means relieving the pressure in the second pilot servo, the relief valve means including a hollow pressurized plunger reacting against a spring means with its interior connected to the second pilot servo, the plunger having a first valve opening therein, the plunger passing axially through the displacement piston with the displacement piston acting as a valving means on the first valve opening, the spring force on the plunger increasing as the plunger follows the piston toward a minimum flow position causing the relieving pressure level to increase as the pump swash plate decreases its displacement whereby the torque output of the pump remains substantially constant at various pump discharge pressure levels.
 2. A torque-compensating control system as set forth in claim 1, and a second valve opening in the plunger so positioned that when the maximum system pressure is attained, the second valve opening opens fluid communication between the interior of the plunger and the displacement cylinder thereby reducing displacement of the pump. 